The starting point is a conventional control concept according to which a control variable of a component of the technical plant is held as closely as possible to the set value of the control variable by means of a PI controller. To do so, the PI controller determines a correction action for an actuator assigned to one of the components from the difference between the set value and actual value of the control variable (control deviation), so that the control variable reaches the required set value as quickly and accurately as possible and at the same time moves as little as possible from this set value in the course of the control action. A strong oscillation of the actual value of the control variable should particularly be avoided so that the control system consisting of the components of the technical plant and the controller does not have a tendency to hunt.
For the control of power station components it is, for example, known for the control parameters of a PI controller used for control, particularly the control ratio and integral-action time, to be set to a constant value in advance in each case and not to be altered again during the control action when operating the plant. The advantage of this is that only a few parameters, particularly the aforementioned controller parameters, have to be set when using a PI controller, and very often no further adjustments have to be made during the control action.
However, the use of parameter values set in advance for the PI controller is not always the optimum solution, particularly for the control of power station plants, because in the course of time during the operation of the power station plant the dynamic behavior of the technical process effected by the components of the technical plant can change. Thus, after a change of this kind in the dynamic behavior of the components the controller parameters set in advance are no longer optimum for the new operating conditions now present and under certain circumstances can even lead to failure of the control system, so that the desired control effect can no longer be achieved. For example, after a change in the dynamic behavior of the controlled system (components) and thus the control circuit formed by the components and the PI controller, the actual value of the control variable can have a tendency towards unwanted oscillations during the control action that can lead to instability of the controlled system. To make a control system of this kind again suitable for use during the operation of the technical plant, it is usually necessary to at least take the affected components of the technical plant out of service, to re-adjust the controller parameters corresponding to the changed dynamic behavior of the controlled system, which possibility renders comprehensive test runs necessary before renewed commissioning, and then to restore the components, including their controller, to service.
Apart from the fact that a controlled circuit of a technical plant that has become functionally unserviceable can be hazardous to operators and potentially damaging to machines, particularly if a control system that has become unstable creates corrections that are too great and/or actual values of the control variable that are far too large, and because of the downtime required to repair the control system a production failure of the technical plant is practically unavoidable. If the technical plant is a power station, the failure of a component of the technical plant can, for example, jeopardize the supply of electrical power to an area.
The use of constant values for the PI controller leads to problems, particularly where used for control in power stations with a steam process, because in power stations of this kind the control systems to be controlled often have a non-linear behavior. A set of parameter values for the PI controller that can be regarded as optimum for an operating state of the controlled components can under a different operating condition of these components produce control results that because of the existing non-linearities are at least of only limited use. Thus if there is an unavoidable shift in the working point of a non-linear components to be controlled or the technical plant during operation of the technical plant, the preset controller parameters are possibly no longer suitable for the new operation point (working point) for producing the desired control effect. This can even have the effect that a PI controller has an optimum control behavior for an operating point of the component, but results in completely inadequate or even dangerous control outcomes when a changeover to a new operating point occurs.